1. Field of the Invention
The present invention relates to a writing optical system in a laser printer, a copy machine, or a laser PPF (Plain Paper Fax), specifically, relates to a multi-beam scanning device that uses a plurality of laser beams for scanning at the same time to obtain high image quality at a high speed.
2. Description of the Related Art
In a laser scanning device of a laser printer, a copy machine, or a laser PPF, it is well-known that a high speed and a high density image can be achieved by using a writing optical system of a laser scanning device in which a plurality of laser beams are made to scan the surface of a medium at the same time. This is the so-called multi-beam scanning technique.
Japanese Unexamined Patent Publications No. 2001-13433 and No. 2000-241730 have disclosed techniques for improving the image quality when such a multi-beam scanning device is employed.
In Japanese Unexamined Patent Publications No. 2001-13433, it is disclosed that the emitting directions of a number of laser beams from a number of light emitting sources are all inclined by the same angle, thereby the laser beams have the same shapes of scanning line bending, and as a result, the fluctuation of scanning line intervals in the sub-scanning direction is reduced, furthermore the density fluctuation can be suppressed to a low level.
However, in the technique disclosed in Japanese Unexamined Patent Publication No. 2001-13433, only the influence of the emission angles of laser beams is considered by placing a limitation on the emission angles, but the influences of other factors are not considered at all, for example, the beam diameter, desired scanning line intervals, and fluctuation of the scanning line intervals are not considered.
On the other hand, in Japanese Unexamined Patent Publication No. 2000-241730, it is disclosed that light transmitting members are arranged in the light path, and by moving the light transmitting members, positions of the laser beams in the sub-scanning direction are corrected.
Turning to the problem to be solved by the present invention, when using a multi-beam writing optical system that uses a plurality of laser beams for scanning, it is different from scanning with a single laser beam in the aspect that in the sub-scanning direction, the laser beams pass through positions deviating from the optical axis of the scanning optical system, which includes, for example, an fθ lens. This situation is explained below in detail with reference to FIG. 2. When such a laser beam deviating from the optical axis of the scanning optical system is directed to scan a medium, for example, a photoconductor, there arises a problem in that the scanning line of the laser beam on the scanning surface of the medium is not straight, but is bent; this is called the “scanning line bending” phenomenon. The amount of scanning line bending increases when the deviation of the laser beam from the optical axis of the scanning optical system becomes large. In other words, in a multi-beam writing optical system, among the number of laser beams that scan the medium at the same time, the more peripheral laser beams, that is, the laser beams farther from the optical axis of the lenses, have greater scanning line bending.
Due to the scanning line bending, the scanning line intervals, that is, the distance between two adjacent scanning lines, in the sub-scan direction, are not uniform. As described later with reference to FIG. 4, near centers of scanning lines, the scanning line intervals are small; at positions other than the centers and ends of scanning lines, the intervals are large.
Because of the fluctuation of the scanning line intervals, the pixel density in an image fluctuates, too. In particular, the scanning line interval is quite large near the centers of two neighboring scanning lines formed by laser beams deflected by two successive deflection reflection surfaces. This interval is roughly twice that of scanning lines of laser beams deflected by the same deflection reflection surface. So, in this region, the density fluctuation in an image is more noticeable.